Monitoring transitions of a circuit

ABSTRACT

A circuit includes a guard trigger circuit that includes a first input node adapted to be coupled to a first state signal, a second input node adapted to be coupled to a second state signal and an output node. The circuit also includes a reset synchronizer circuit that includes an input node coupled to the output node of the guard trigger circuit, a clock node adapted to be coupled to a clock signal and an output node. The circuit further includes a timeout circuit including an input node coupled to the output node of the reset synchronizer circuit, a clock node adapted to be coupled to the clock signal and an output node. The circuit still further includes a reset requestor circuit that includes a first input node coupled to the output node of the guard trigger circuit, a second node coupled to the output node of the timeout circuit.

TECHNICAL FIELD

This relates generally to electronic circuitry, and more particularly tomonitoring transitions of a circuit.

BACKGROUND

In the fields of electronics and computer science a finite state machine(FMS) refers to a computation model that implemented with hardware orsoftware that is employable to simulate sequential logic and/orexecution of certain computer programs. FSMs are employable to modelproblems in many fields including mathematics, artificial intelligence,games, and linguistics.

An FSM implemented in a digital circuit can include, but is not limitedto a programmable logic device, a programmable logic controller, logicgates, flip-flops and/or relays. In such a situation a programmablecomponent of the FSM stores state variables and combinational logic todetermine a state transition and an output of the FSM. Additionally, insome examples, an FSM can be instantiated on a computing platform, suchas a general-purpose computer or a controller.

SUMMARY

In a first example, a circuit includes a guard trigger circuit thatincludes a first input node adapted to be coupled to a first statesignal, a second input node adapted to be coupled to a second statesignal and an output node adapted to provide a trigger signal. The guardtrigger circuit is adapted to determine a state transition in progressbased on the first state signal and the second state signal and providethe trigger signal based on whether the state transition is in progress.The circuit can include a reset synchronizer circuit that includes aninput node coupled to the output node of the guard trigger circuit, aclock node adapted to be coupled to a clock signal and an output nodeadapted to provide a synchronization signal based on the trigger signal.The circuit can further include a timeout circuit that includes an inputnode coupled to the output node of the reset synchronizer circuit, aclock node adapted to be coupled to the clock signal; and an output nodeadapted to provide a timeout signal based on the synchronization signal.The circuit also includes a reset requestor circuit including a firstinput node coupled to the output node of the guard trigger circuit, asecond node coupled to the output node of the timeout circuit and anoutput node adapted to provide a reset signal based on the triggersignal and the timeout signal.

In a second example, a method includes asserting a trigger signal inresponse to detecting that a transition in a circuit is in progress. Themethod also includes asserting a synchronization signal in response todetecting the trigger signal being asserted for a first number of cyclesin a clock signal. The method further includes asserting a timeoutsignal in response to detecting the synchronization signal beingasserted for a second number of cycles in the clock signal. The methodalso includes asserting a reset signal in response to detecting thetimeout signal and the trigger signal being asserted concurrently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a guardian circuit module that isemployable to monitor operations of an external circuit.

FIG. 2 is a circuit diagram of a guardian circuit module for an FSM.

FIG. 3 is an example of a circuit that includes a guardian circuitmodule that monitors transitions of states in an FSM.

FIG. 4 is an example of a state diagram of an FSM.

FIG. 5 is a flowchart of an example method for monitoring operations ofan external circuit.

DETAILED DESCRIPTION

This disclosure relates to a guardian circuit (e.g., a sub-circuit) foran external circuit, such as a finite state machine (FSM) implemented ona circuit (e.g., an IC chip). Some FSMs switch states on a rapid basisover a long period of time. For instance, in one example, a flow metercircuit might switch between a first state that measures flow and asecond state that writes a value to memory characterizing the measuredflow. In such situations, throughout a lifetime of the FSM, the FSM mayget stuck between state transitions. For instance, in the example of aflow meter circuit, the transition between the first state (measuringthe flow, an analog operation) and the second state (writing to memory,a digital operation) may include switching power modes. Such statetransitions might slow down or hang-up operations of the circuit if thestate transitions are not completed as expected. Moreover, area andpower constraints might not provide sufficiently complex solutions tosuch problems.

The guardian circuit module receives a guardian clock signal thatoperates independently of other clock signals within the circuit. Toincrease the robustness of such a circuit, the guardian circuit detectsa start of a state transition (e.g., based on a state signal and a nextstate signal) in the FSM and responds if a given period of time elapsesbefore the state transition completes. In some examples, in response todetecting the state transition, the guardian circuit module asserts atrigger signal. The guardian circuit module is configured to assert areset signal in response to the trigger signal being asserted for athreshold number of cycles in the guardian clock signal. The resetsignal is employable to reset the FSM to a known safe state, such as aninitial power-on state. Conversely, if the state transition is completedprior to the guardian clock signal cycling the threshold number ofcycles, the trigger signal is de-asserted, and the reset signal is notasserted. In this manner, if a state transition takes too long (morethan the threshold cycle count of the guardian clock signal), theguardian circuit module asserts the reset signal, which causes the FSMto reset.

FIG. 1 is a block diagram of a guardian circuit module 100 implementedas a circuit, such as a sub-circuit that is employable to monitoroperations of an external circuit 103, implemented on a circuit 102,such as a system on a chip (SoC). The external circuit 103 isimplemented, for example, by a controller operating on the SoC. In sucha situation, the guardian circuit module 100 is implemented as asub-circuit on the SoC. The guardian circuit module 100 is configured toassert a reset signal, RESET based on a status of input signals. Thereset signal, RESET is implemented as a high priority reset signal.Accordingly, components (e.g., such as the external circuit 103) of thecircuit 102 are reset and restored to a safe state, such as an initialpower-on state in response to the reset signal, RESET being asserted.

In some examples, the external circuit 103 is implemented as an FSM. Inother examples, the external circuit 103 is implemented as a circuit tofacilitate handshaking. In still other examples, the external circuit103 is implemented as a power module. For purposes of simplification ofexplanation, examples are described wherein the external circuit 103 isimplemented as an FSM. In any of these examples, and other examples, theexternal circuit 103 may have transitions (e.g., state transitions,signal transitions, feedback return signals, etc.) that could cause theexternal circuit 103 to delay or hang.

More particularly, in some examples, the external circuit 103 isimplemented as a software defined FSM of the circuit 102, particularlyin situations where the circuit 102 is implemented as an SoC with anembedded controller. In other examples, the external circuit 103 isimplemented as a hardware module within the circuit 102. In eithersituation (and other situations), there may be race conditions and/orpriority hierarchies that cause state transitions in the FSM to delay orhang. Additionally, even in situations where the external circuit 103 isrelatively simple, power sags (e.g., brownouts) could causesub-threshold voltage levels at the external circuit 103, and suchsub-threshold voltage levels may delay or prevent a transition fromcompleting. For instance, in a situation where a transition includeschanging power modes, a power sag could impede the external circuit 103,implemented as an FSM, from completing a transitioning from a state witha low power mode to a state with a high power mode.

The guardian circuit module 100 receives a dedicated clock signal, GUARDCLK, which can alternatively be referred to as a guardian clock signal.In some examples, the dedicated clock signal, GUARD CLK, is implementedas a clock signal that is independent of other clock signals employed onthe circuit 102. In some examples, the guardian circuit module 100includes components (e.g., a clock generator) for generating theguardian clock, GUARD CLK. In other examples, the guardian clock, GUARDCLK is generated by a component of the circuit 102 external to theguardian circuit module 100. The dedicated clock signal, GUARD CLK has aclock counter interval that is longer than a maximum permitted time fora longest transition to complete in the external circuit 103. Forinstance, in some examples, the dedicated clock signal, GUARD CLK isthree orders of magnitude or more longer than a counter interval of asystem clock signal of the circuit 102 that controls operations of theexternal circuit 103. For instance, in some examples if the system clocksignal drives operations on the external circuit 103, and has a counterinterval of about 1 microsecond (μs), the dedicated clock signal, GUARDCLK would have a counter interval of about 1 millisecond (ms) or more.In this manner, the dedicated clock signal, GUARD CLK is slower than thesystem clock signal.

The guardian circuit module 100 includes a guard trigger circuit,referred to as a guard trigger 104. The guard trigger 104 is configuredto assert a trigger signal, TRIGGER_N based on a status of inputsignals. In particular, the guard trigger 104 receives a state signal,STATE, and a next state signal, NEXT STATE that characterizes a presentstate of the external circuit 103, implemented as an FSM and a nextstate of the external circuit 103, respectively. The state signal,STATE, and the next state signal, NEXT STATE represent signals that aregenerated based on a comparison of state vectors for the externalcircuit 103. The guard trigger 104 also receives a brownout signal,BROWNOUT that characterizes a power status of the circuit 102. Thebrownout signal, BROWNOUT is asserted, for example, in situations wherethere is insufficient power to operate the circuit 102 and/or thecircuit 102 is being reset. In some examples, the guard trigger 104 alsoreceives a guard enable signal, GUARD ENABLE that is asserted to enableoperation of the guard trigger 104.

In the examples described herein, the term “asserted” (and itsderivatives) in reference to a given signal refers to the given signalbeing assigned a logical 1 or a logical 0 depending on the type of thegiven signal. More particularly, if the given signal is an active highsignal, assertion of the given signal sets the given signal to alogical 1. Additionally, if the given signal is an active low signal,assertion of the given signal sets the given signal to a logical 0.Conversely, the term “de-asserted” (and its derivatives) in reference tothe given signal indicates that the given signal is assigned theopposite logical value from the logical value the given signal isassigned to assert the given signal. For instance, if the given signalis an active high signal, de-asserting the given signal sets the givensignal to a logical 0. Additionally, if the given signal is an activelow signal, de-asserting the given signal sets the given signal to alogical 1. Moreover, it is understood that in examples where a givensignal is described as being active high or active low, respectively, inother examples, the given signal is implemented as active low or activehigh, respectively.

In some examples, the guard trigger is configured to assert the triggersignal, TRIGGER_N in response to detecting a state transition is inprogress at the external circuit 103. The state transition is indicatedby a status of the state signal, STATE and the next state signal, NEXTstate. More particularly, if the state signal, STATE is asserted and thenext state signal, NEXT STATE is not asserted, or vice versa, the guardtrigger 104 determines that the external circuit 103 is in a transition(e.g., a state transition), and the guard trigger 104 asserts thetrigger signal, TRIGGER_N. Conversely, if the state signal, STATE andthe next state signal, NEXT STATE are both asserted, the guard trigger104 de-asserts the trigger signal, TRIGGER_N.

Additionally, a number of conditions can cause the guard trigger toinhibit (prevent) assertion of the trigger signal, TRIGGER_N. Inparticular, the guard trigger 104 is configured such that if thebrownout signal, BROWNOUT is asserted, the guard trigger 104 de-assertsthe trigger signal, TRIGGER_N. The brownout signal, BROWNOUT is assertedin situations where the circuit 102 is being reset, and components,including the external circuit 103 are being set to a safe state, suchas an initial, power-on state. The guard trigger 104 is configured tode-assert the trigger signal, TRIGGER_N to allow for the externalcircuit 103 of the circuit 102 to be set to the initial, power on statewithout interruption.

Similarly, the guard trigger 104 is configured to inhibit assertion ofthe trigger signal if the guard enable signal, GUARD ENABLE isde-asserted. De-assertion of the guard enable signal, GUARD ENABLEindicates that the operations of the guardian circuit module 100 are tobe disabled.

In some examples, the guard trigger 104 maintains the trigger signal,TRIGGER_N in the asserted state until (i) the state signal, STATE andthe next state signal, NEXT STATE are asserted, (ii) the brownoutsignal, BROWNOUT is asserted or (iii) the guard enable signal, GUARDENABLE is de-asserted. In response to detection one of the events(i)-(iii), the guard trigger 104 de-asserts the trigger signal,TRIGGER_N. In other examples, more or fewer events than events (i)-(iii)cause the guard trigger 104 to de-assert the trigger signal, TRIGGER_N.

The trigger signal, TRIGGER_N is provided to a reset synchronizercircuit, referred to as a reset synchronizer 108 and to a resetrequestor circuit, referred to as a reset requestor 112. The resetsynchronizer 108 also receives the dedicated clock signal, GUARD CLK asan input. The reset synchronizer 108 is configured to assert asynchronization signal, SYNC_N if the trigger signal, TRIGGER_N isasserted for a predetermined number of cycles of the dedicated clocksignal, GUARD CLK. In some examples, the reset synchronizer 108 isconfigured as a shift register. As one example, the reset synchronizer108 is configured to assert the synchronization signal, SYNC_N inresponse to the trigger signal, TRIGGER_N being asserted for at leasttwo cycles of the dedicated clock signal, GUARD CLK. In other examples,the reset synchronizer 108 is configured to assert the synchronizationsignal, SYNC_N after the trigger signal, TRIGGER_N has been asserted formore than two or less than two cycles of the dedicated clock signal,GUARD CLK.

The synchronization signal, SYNC_N is provided to a timeout circuit 116.The timeout circuit 116 also receives the dedicated clock signal, GUARDCLK. The timeout circuit 116 is configured to assert a timeout signal,TIMEOUT if the synchronization signal, SYNC_N is asserted for apredetermined number of cycles of the dedicated clock signal, GUARD CLK.In some examples, the timeout circuit 116 is configured as a shiftregister. As one example, the timeout circuit 116 is configured toassert the timeout signal, TIMEOUT in response to the synchronizationsignal, SYNC_N being asserted for at least three cycles of the dedicatedclock signal, GUARD CLK. In other examples, the timeout circuit 116 isconfigured to assert the timeout signal, TIMEOUT after thesynchronization signal, SYNC_N has been asserted for more than three orless than three cycles of the dedicated clock signal, GUARD CLK.

The timeout signal, TIMEOUT and the trigger signal, TRIGGER_N areprovided to the reset requestor 112. The reset requestor 112 generates areset signal, RESET if both the trigger signal, TRIGGER_N and thetimeout signal, TIMEOUT are asserted. As noted, the reset signal, RESETis implemented as a high-priority request signal that causes the circuit102 to reset the external circuit 103 to an initial power-on state (orsome other safe state).

By implementing the guardian circuit module 100, a relatively simple androbust state transition monitoring circuit is provided for the externalcircuit 103. In particular, the guardian circuit module 100 asserts thereset signal, RESET in situations where the external circuit 103 delaysbeyond a threshold number of cycles of the dedicated clock signal, GUARDCLK (e.g., for 5 cycles of the dedicated clock signal, GUARD CLK)between state transitions. By asserting the reset signal, RESET in thismanner, the guardian circuit module 100 increases the robustness of thecircuit 102. Moreover, the guardian circuit module 100 obviates the needfor a high-power, complex monitor of the transitions of the externalcircuit 103.

FIG. 2 is a circuit diagram of a guardian circuit module 200 for an FSM.The guardian circuit module 200 is employable to implement the guardiancircuit module 100 of FIG. 1 . For purposes of simplification ofexplanation, the same signal and component names are employed in FIGS. 1and 2 to denote the same signal or component.

The guardian circuit module 200 is implemented as a module of a circuit,such as the circuit 102 of FIG. 1 . Unless otherwise noted, componentsof the guardian circuit module 200 are operable at sub-threshold levels.Thus, in situations where insufficient power is available to operate theFSM, the guardian circuit module 200 can still operate in the expectedmanner. The guardian circuit module 200 includes a guard trigger 204that is employed to implement the guard trigger 104 of FIG. 1 . Theguard trigger 204 receives inputs characterizing a state of an FSMimplemented on the circuit, such as the external circuit 103 of FIG. 1 .More particularly, the guard trigger 204 receives a state signal, STATEand a next state signal, NEXT STATE that characterize a state of theFSM. In some examples, the state signal, STATE and a next state signal,NEXT STATE characterizes a comparison of state vectors for the FSM.

The state signal, STATE and a next state signal, NEXT STATE are coupledto inputs to an XNOR gate 208 of the guard trigger 204. The XNOR gate208 outputs a state transition trigger signal, STATE TRANSITIONTRIGGER_N to an input of an OR gate 212 of the guard trigger 204.

The term “couple” (and derivatives) is used throughout thespecification. The term may cover connections, communications, or signalpaths that enable a functional relationship consistent with thedescription of this disclosure. For example, if device A generates asignal to control device B to perform an action, in a first exampledevice A is coupled to device B, or in a second example device A iscoupled to device B through intervening component C if interveningcomponent C does not substantially alter the functional relationshipbetween device A and device B such that device B is controlled by deviceA through the control signal generated by device A.

A brownout signal, BROWNOUT is coupled to the OR gate 212, and a guardenable signal, GUARD ENABLE is coupled to an inverted input of the ORgate 212. In the illustrated example, the brownout signal, BROWNOUT andthe guard enable signal, GUARD ENABLE are active high signals. Assertionof the brownout signal, BROWNOUT indicates that the FSM is being resetto a safe state, such as an initial power-on state. For instance, thebrownout signal, BROWNOUT is asserted in situations where power to theFSM drops below a threshold level.

The OR gate 212 outputs a trigger signal, TRIGGER_N. In the exampleillustrated, the trigger signal, TRIGGER_N is an active low signal. Theguard trigger 204 asserts the trigger signal (logical 0), TRIGGER_N inresponse to the state transition trigger signal, STATE TRANSITIONTRIGGER_N being de-asserted, the brownout signal, BROWNOUT (logical 0)being de-asserted and the guard enable signal, GUARD ENABLE (logical 1)being asserted.

The trigger signal, TRIGGER_N is coupled to an input of a resetsynchronizer 216. A dedicated clock signal, GUARD CLK is coupled to aclock input of the reset synchronizer 216. The dedicated clock signal,GUARD CLK, has counter interval that is longer than a maximum time for alongest transition in the FSM. For instance, in some examples, thededicated clock signal, GUARD CLK has a counter interval that is threeorders of magnitude longer than a system clock signal of the circuitthat is employed to drive operations of the FSM. For instance, if thesystem clock signal has a counter interval of about 100 nanoseconds(ns), the dedicated clock signal, GUARD CLK has a counter interval ofabout 100 μs. In other examples, the difference between the counterinterval of the system clock signal and the dedicated clock signal,GUARD CLK are different. The reset synchronizer 216 is configured toassert a synchronization signal, SYNC_N (logical 0) if the triggersignal, TRIGGER_N is asserted (logical 0) for at least two cycles of thededicated clock signal, GUARD CLK. In the example illustrated, thesynchronization signal, SYNC_N is an active low signal. Conceptually,the reset synchronizer 216 receives an asynchronous signal, namely, thetrigger signal, TRIGGER_N and outputs a signal synchronized with thededicated clock signal, GUARD CLK, namely the synchronization signalSYNC_N.

In some examples, the reset synchronizer 216 is configured as a shiftregister. More particularly, in the example illustrated, the resetsynchronizer 216 is implemented with two cascading D flip-flops, namelya first D flip-flop 220 and a second D flip-flop 224. The triggersignal, TRIGGER_N is coupled to a preset input, PR of the first Dflip-flop 220 and the second D flip-flop 224. The dedicated clocksignal, GUARD CLK is coupled to a clock input of the first D flip-flop220 and the second D flip-flop. A tie low input 228 (e.g., about 0Volts) is coupled to a data input, D of the first D flip-flop 220 toapply a logical 0 to the data input, D of the first D flip-flop 220. Anon-inverted output, Q of the first D flip-flop 220 is coupled to a datainput, D of the second D flip-flop 220. A non-inverted output, Q of thesecond D flip-flop 224 outputs the synchronization signal, SYNC_N of thereset synchronizer 216.

The synchronization signal, SYNC_N is coupled to an input of a timeoutcircuit 232. Additionally, the dedicated clock signal, GUARD CLK iscoupled to a clock input of the timeout circuit 232. The timeout circuit232 is configured to assert a timeout signal, TIMEOUT (logical 1) if thesynchronization signal, SYNC_N is asserted (logical 0) for three or morecycles of the dedicated clock signal, GUARD CLK. In the exampleillustrated, the timeout signal, TIMEOUT is an active high signal.

In some examples, the timeout circuit 232 is implemented as a shiftregister. More particularly, the timeout circuit 232 may be implementedwith three cascaded D flip-flops, namely a first D flip-flop 236, asecond D flip-flop 240 and a third D flip-flop 244. In other examples,the timeout circuit 232 is implemented with a different number of Dflip-flops.

The dedicated clock signal, GUARD CLK is coupled to a clock input of thefirst D flip-flop 236, the second D flip-flop 240 and the third Dflip-flop 244 of the timeout circuit 232. The synchronization signal,SYNC_N is coupled to a clear input, CL of the first D flip-flop 236, thesecond D flip-flop 240 and the third D flip-flop 244 of the timeoutcircuit 232. The timeout circuit 232 includes a tie high input 248 thatasserts a logical 1 on a data input, D of the first D flip-flop 236.

A non-inverted output, Q of the first D flip-flop 236 is coupled to adata input, D of the second D flip-flop 240. Similarly, a non-invertedoutput, Q of the second D flip-flop 240 is coupled to a data input, D ofthe third D flip-flop 244. Moreover, a non-inverted output, Q of thethird D flip-flop 244 provides the timeout signal, TIMEOUT.

The timeout signal, TIMEOUT is coupled to an input of a reset requestor250. The trigger signal, TRIGGER_N is also coupled to an input of thereset requestor 250. The reset requestor 250 asserts a reset signal,RESET (logical 1) in response to the timeout signal, TIMEOUT and thetrigger signal, TRIGGER_N being asserted concurrently. In theillustrated example, the reset signal, RESET is an active high signal.

In the illustrated example, the reset requestor 250 is implemented withan AND gate 254 with an inverted input. In other examples, the resetrequestor 250 is implemented with more logical gates and/or othercircuit components. The trigger signal, TRIGGER_N is coupled to theinverted input of the AND gate 254. The timeout signal, TIMEOUT iscoupled to a non-inverted input of the AND gate 254.

In the example illustrated, the state signal, STATE and the next statesignal, NEXT STATE are active high signals and the state transitiontrigger signal, STATE TRANSITION TRIGGER_N is an active low signal. Insituations where the FSM is remaining in the same state, both the statesignal, STATE and the next state signal, NEXT STATE are asserted(logical 1). In situations where the FSM is transitioning betweenstates, either the state signal, STATE or the next state signal, NEXTSTATE is de-asserted (logical 0). If either (or both) of the statesignals, STATE or the next state signal, NEXT STATE are de-asserted(logical 0), the state transition trigger signal, STATE TRANSITIONTRIGGER_N is asserted (logical 0). The guard trigger 204 asserts thetrigger signal (logical 0), TRIGGER in response to the state transitiontrigger signal, STATE TRANSITION TRIGGER_N (logical 0) being asserted,the brownout signal, BROWNOUT (logical 0) being de-asserted and theguard enable signal, GUARD ENABLE (logical 1) being asserted.Conversely, the guard trigger 204 de-asserts the trigger signal,TRIGGER_N (logical 1) in response the state transition trigger signal,STATE TRANSITION TRIGGER_N (logical 1) being de-asserted, the brownoutsignal, BROWNOUT (logical 1) being asserted or the guard enable signal,GUARD ENABLE (logical 0) being de-asserted.

The trigger signal, TRIGGER_N (logical 1) being de-asserted causes thefirst D flip-flop 220 and the second D flip-flop 224 on the resetsynchronizer 216 to operate in a preset mode. In the preset mode, boththe first D flip-flop 220 and the second D flip-flop 220 output alogical 1 on the respective non-inverted output, Q. Thus, if the triggersignal, TRIGGER_N is de-asserted (logical 1), the synchronizationsignal, SYNC_N is also de-asserted (logical 1).

The synchronization signal, SYNC_N (logical 1) being de-asserted, causesthe first D flip-flop 236, the second D flip-flop 240 and the third Dflip-flop 244 of the timeout circuit 232 to operate in a clear mode. Inthe clear mode, the first D flip-flop 236, the second D flip-flop 240and the third D flip-flop 244 of the timeout circuit 232 output alogical 0 on the respective non-inverted outputs, Q. As noted, thenon-inverted output Q of the third D flip-flop 244 of the timeoutcircuit 232 provides the timeout signal, TIMEOUT that is coupled to theAND gate 254 of the reset requestor 250 at a non-inverted input, Q.Additionally, the trigger signal, TRIGGER_N is coupled to the invertedinput of the AND gate 254 of the reset requestor 250. The timeoutsignal, TIMEOUT (logical 0) being de-asserted and the trigger signal,TRIGGER_N (logical 1) being de-asserted causes the AND gate 254 tode-assert the reset signal, RESET (logical 0). Accordingly, the triggersignal, TRIGGER_N (logical 1) being de-asserted causes the reset signal,RESET to be de-asserted (logical 0).

Conversely, the trigger signal, TRIGGER_N (logical 0) being asserted,causes the first D flip-flop 220 and the second D flip-flop 224 of thereset synchronizer 216 to operate in shift mode. In shift mode, thelogical 0 applied by the tie low input 228 is released into the cascadeof D flip-flops of the reset synchronizer 216. In such a situation, thenon-inverted output, Q of the first D flip-flop 220 of the resetsynchronizer 216 is set to a logical 0 in response to a next pulse ofthe dedicated clock signal, GUARD CLK. Subsequently, in response toreceipt of the logical 0 at the data input, D of the second D flip-flop224, the second D flip-flop outputs a logical 0 on the non-invertedoutput, Q in response to a next pulse of the dedicated clock signal,GUARD CLK. As noted, the non-inverted output, Q on the second Dflip-flop 224 of the reset synchronizer 216 provides the synchronizationsignal, SYNC_N to the timeout circuit 232, such that the synchronizationsignal, SYNC_N is asserted (logical 0).

In summary, the reset synchronizer 216 is configured to assert thesynchronization signal, SYNC_N (logical 0) if the trigger signal,TRIGGER_N is asserted (logical 0) for at least two cycles of thededicated clock signal, GUARD CLK. Additionally, if the trigger signal,TRIGGER_N is de-asserted (logical 1) prior to completion of the at leasttwo clock cycles of the dedicated clock signal, GUARD CLK, the first Dflip-flop 220 and the second D flip-flop 224 both transition to thepreset mode, and the synchronization signal, SYNC_N is de-asserted(logical 1).

Assertion of the synchronization signal, SYNC_N (logical 0) sets theclear input, CL of the first D flip-flop 236, the second D flip-flop 240and the third D flip-flop 244 of the timeout circuit 232 to a logical 0.In response, each of the first D flip-flop 236, the second D flip-flop240 and the third D flip-flop 244 of the timeout circuit 232 operate ina shift mode. In the shift mode, the tie high input 248 applies alogical 1 on the data input, D of the first D flip-flop 236. In responseto a next pulse of the dedicated clock signal, GUARD CLK, the first Dflip-flop 236 outputs a logical 1 on the non-inverted output Q, which iscoupled to the data input, D of the second D flip-flop 240.

Subsequently, if the data input, D of the second D flip-flop 240 is setto a logical 1, the second D flip-flop 240 outputs a logical 1 on thenon-inverted output Q of the second D flip-flop 240 in response to anext clock pulse of the dedicated clock signal, GUARD CLK. As noted, thenon-inverted output Q of the second D flip-flop 240 is coupled to thedata input, D of the third D flip-flop 244. If the data input, D of thethird D flip-flop 244 is set to a logical 1, the third D flip-flop 244outputs a logical 1 on the non-inverted output Q of the third Dflip-flop 244 in response to a next clock pulse of the dedicated clocksignal, GUARD CLK. As noted, the non-inverted output, Q of the third Dflip-flop 244 is the timeout signal, TIMEOUT.

In summary, the timeout circuit 232 is configured to assert the timeoutsignal, TIMEOUT (logical 1) if the synchronization signal, SYNC_N isasserted (logical 0) for at least three cycles of the dedicated clocksignal, GUARD CLK. Additionally, if the synchronization signal, SYNC_Nis de-asserted (logical 1) prior to completion of the at least threeclock cycles of the dedicated clock signal, GUARD CLK, the first Dflip-flop 236, the second D flip-flop 240 and the third D flip-flop 244transition to the clear mode, and the timeout signal, TIMEOUT isde-asserted (logical 0). In response to the timeout signal, TIMEOUT(logical 1) being asserted and the trigger signal, TRIGGER_N (logical 0)being asserted, the reset requestor 250 asserts the reset signal, RESET.

Taken together, the reset synchronizer 216, the timeout circuit 232 andthe reset requestor 250 assert the reset signal, RESET (logical 1) ifthe trigger signal, TRIGGER_N remains asserted (logical 0) for athreshold number of cycles of the dedicated clock signal, GUARD CLK(e.g., at least five cycles of the guardian clock cycle, GUARD CLK).

In some examples, the reset signal, RESET is a high priority resetrequest. Thus, in response to the RESET request being asserted, the FSMis set to an initial-power on state, or other safe state. Accordingly,the guardian circuit module 200 is a relatively simple and robust statetransition monitor circuit for the FSM. In particular, the guardiancircuit module 200 asserts the reset signal, RESET in situations wherethe FSM delays too long (e.g., for 5 cycles of the dedicated clocksignal, GUARD CLK) between state transitions. The guardian circuitmodule 200 obviates the need for a high-power, complex monitor of thestate transitions of the FSM.

FIG. 3 illustrates an example of a circuit 300 (e.g., an SoC) thatincludes a guardian circuit module 304 that monitors transitions ofstates in an FSM 308. The circuit 300 is employable to implement thecircuit 102 of FIG. 1 . More particularly, the guardian circuit module304 is employable to implement the guardian circuit module 100 of FIG. 1and/or the guardian circuit module 200 of FIG. 2 .

In the present example, the FSM 308 operates as a software instantiatedFSM operating on a controller 312 of the circuit 300. In some examples,the controller 312 is implemented as a computing platform that includesa general-purpose processor with embedded instructions for implementingthe FSM 308. In various examples, the FSM 308 is implemented as a Mealystate machine, a Moore state machine or a Medvedev state machine or acombination thereof.

In the example illustrated, the controller 312 communicates with asensor 316. In some examples, the sensor 316 is an analog sensor, suchas a flow meter, a temperature sensor, an accelerometer, etc. In anysuch situation, it is presumed that the FSM 308 controls operations ofthe sensor 316. For instance, in one example, in a given state the FSM308 causes the sensor 316 to measure an external condition (e.g., flow,temperature, acceleration, etc.) and in another state, the FSM 308writes data characterizing the measured condition to a non-transitorymemory of the controller 312. In such an example, in a given state, theFSM 308 operates in a high power mode, and in the other state, the FSM308 operates in a low power mode. Accordingly, in this example,switching between the given state and the other state includes switchingpower modes. In some instances, switching between power modes impedes orprevents the FSM 308 from completing a state transition.

The controller 312 includes an FSM state logic module 320. The FSM statelogic module 320 is implemented as a gate network that monitors a stateof the FSM 308. As an example, the FSM state logic module 320 comparesstate vectors of the FSM 308 to generate a state signal, STATEcharacterizing a current state of the FSM 308 and a next state signal,NEXT STATE characterizing a next state of the FSM 308. The state signal,STATE and the next state signal, NEXT STATE are provided to the guardiancircuit module 304.

In some examples, the controller 312 provides a brownout signal,BROWNOUT. The brownout signal, BROWNOUT characterizes a power status ofthe circuit 300. The brownout signal, BROWNOUT is asserted, for example,in situations where there is insufficient power to operate the circuit300 and/or the FSM 308 is being reset. In some examples, the controller312 also provides the guardian circuit module 304 with a guard enablesignal, GUARD ENABLE that is asserted to enable operation of theguardian circuit module 304.

The circuit 300 also includes a clock generator 324 that generates adedicated clock signal, GUARD CLK. The dedicated clock signal, GUARD CLKoperates independently of other clock signals in the circuit, including,but not limited to a system clock signal, SYS CLK that is generated by asystem clock generator 326 (or other component) of the controller 312that controls operations of the FSM 308. The dedicated clock signal,GUARD CLK has a counter interval that is longer than a maximumtransition time for a state transition of the FSM 308. In some examples,the dedicated clock signal, GUARD CLK has a cycle time (period) that isthree orders of magnitude (or more) longer than the cycle time of thesystem clock signal, SYS CLK. Thus, if the system clock signal, SYS CLKhas a cycle time of 20 μs, the dedicated clock signal, GUARD CLK has acycle time of 20 ms or longer. In other examples, other relationshipsbetween the cycle time of the dedicated clock signal, GUARD CLK, and thesystem clock signal, SYS CLK are selected.

The guardian circuit module 304 is configured to assert a reset signal,RESET in certain conditions, as described herein. More particularly, theguardian circuit module 304 is configured to assert the reset signal,RESET in situations where the guard enable signal, GUARD ENABLE isasserted, the brownout signal, BROWNOUT is de-asserted, and the statesignal, STATE and the next state signal, NEXT STATE indicate that theFSM 308 is in a state transition for at least a threshold number ofcycles (e.g., five clock cycles) of the dedicated clock signal, GUARDCLK. The reset signal, RESET is provided to the controller 312. Inresponse to assertion of the reset signal, RESET, the controller 312commands the FSM 308 to transition to a safe state, such as an initialpower on state.

FIG. 4 illustrates a state diagram 400 that depicts an example ofpossible states for the FSM 308 of FIG. 3 . The state diagram 400illustrates four possible states, namely, state 0, S0, state 1, S1,state 2, S2 and state 3, S3. Additionally, it is presumed that the statemachine also includes an end state, E0 that is not shown. The design ofstate machines, including the FSM implemented by the state diagram 400often rely on inaccurate assumptions. In particular, some such FSMspresume that time only elapses when the FSM is in a discrete state,namely when the FSM is in state 0, S0 state 1, S1 state 2, S2 or state3, S3. Additionally, FSMs are often designed with the inaccuratepresumption that state transitions are logically instantaneous, suchthat only actions taken during a transition are the setting of flag andvariables and/or the sending of signals. In such a situation, theseactions are taken before the FSM enters the next state. Further still,FSMs are often designed with the presumption that each time a state isentered, the actions of that state are started. Accordingly, a statetransition that points back to the same state causes actions to berepeated from the beginning of entry into the state, and that eachaction started upon entry into a state completes before any tests aremade to exit the state. Such presumptions simplify design of the FSM,but can lead to metastable conditions that can impede or prevent the FSMfrom completing a transition from one state to another.

In the state diagram 400, it is presumed that the FSM includes variablesthat impact a state of the FSM. In the present example, such variablesinclude, a reset variable, RESET, an up-count variable, UPCNT, adown-count variable, DNCNT, an enable variable, EN and an error variableERR. Additionally, in the state diagram 400, operations are listed in avertical axis of each state according to a priority of the operation.Thus, if two operations are possible, based on the state of thevariables, the operation with the highest priority on a respective stateis executed, and the lower priority operation is not executed. Thus, thepriority of operations impacts the state transitions, as describedherein.

In state 0, S0, at 404, the FSM detects that the reset variable, RESEThas been set to a logical 1. In such a situation, no matter the previousor current state, the FSM transitions to state 0, S0, indicated by thestate vector, ALL:S0. State 0, S0 is the initial power-on state wherevariables are set to known values. In the example illustrated in FIG. 4, it is presumed that assertion of the reset signal, RESET illustratedin FIG. 3 sets the reset variable, RESET to a logical 1. Additionally,in state 0, S0, at 408 if the up-count variable, UPCNT and the enablevariable, EN are both set to true (logical 1), the FSM transitions tostate 1, S1, as indicated by the state vector S0:S1. Further, in state0, S0 at 412 if the down-count variable, DNCNT or the error variable,ERR are set to true (logical 1), the FSM stays in state 0, S0 asindicated by the state vector, S0:S0. Additionally, in state 0, S0 it ispresumed that the up-count variable, UPCNT has a higher priority thanthe down-count variable, DNCNT, when both the up-count variable, UPCNTand the down-count variable, DNCNT are true (logical 1). Thus, if boththe up-count variable, UPCNT and the down-count variable, DNCNT are true(logical 1) the operation at 408 is executed, and the FSM transitions tostate 1, S1.

In state 1, S1, at 416, if the down-count variable, DNCNT is true(logical 1), the FSM transitions to state 0, S0, as indicated by thestate vector S1:S0. Additionally in state 1, S1, at 420, if the up-countvariable, UPCNT and the enable variable, EN are true (logical 1) the FSMtransitions to state 2, S2, as indicated by the state vector, S1:S2. Instate 1, S1 it is presumed that the down-count variable, DNCNT has ahigher priority than the up-count variable, UPCNT. Accordingly, if boththe up-count variable, UPCNT and the down-count variable, DNCNT are true(logical 1) the operation at 416 is executed, and the FSM transitions tostate 0, S0.

In state 2, S2 at 424, if the up-count variable, UPCNT is true (logical1), the FSM transitions to state 3, S3, as indicated by the state vectorS2:S3. In state 2, S2, at 428 if the down-count variable, DNCNT is true(logical 1), the FSM transitions to state 1, S1, as indicated by thestate vector S2:S1. In state 2, at operation 432, if the up-countvariable, UPCNT and the enable variable, EN are true (logical 1), theFSM transitions to state E0 (not shown), as indicated by the statevector, S2:E0. However, in state 2, S2 the state transition to E0 atoperation 432 is not executed because in both operations 424 and 432,the up-count variable, UPCNT is set to true (logical 1) and theoperation at 424 has a higher priority than the operation at 432.Accordingly, the operation at 432 is presumed to be superfluous.

In state 3, S3, at 436, at operation 436, if the up-count variable,UPCNT and the enable variable, EN are true (logical 1), the FSMtransitions to state E0 (not shown). In state 3, S3, at 440, if thedown-count variable, DNCNT is true (logical 1), the FSM transitions tostate 2, as indicated by the state vector S3:S2. In state 3, S3 atoperation 444, if the up-count variable, UPCNT is true (logical 1), theFSM transitions to the end state, E0, as indicated by the state vectorS3:E0. Moreover, in state 3 because the operation at 436 has a higherpriority than the operation at 444, the operation at 444 would only beexecuted if the enable variable, EN is false (logical 0) and theup-count variable, UPCNT is true (logical 1).

As demonstrated by the state diagram 400, even a simple FSM can havecomplexities that can lead to unexpected behavior, particularly if theFSM is designed without full regard to priorities of operation.Moreover, if power to the FSM goes to a sub-threshold level, some of thevariables may be set to a metastable state, which can impede or preventthe FSM from completing the transition between states.

In the state diagram 400, it is presumed that a guardian circuit module,such as the guardian circuit module 100 of FIG. 1 , the guardian circuitmodule 200 of FIG. 2 , and/or the guardian circuit module 304 of FIG. 3is configured to monitor transitions between the states, S0-S3. In sucha situation, the guardian clock module asserts a trigger signal inresponse to detecting a state transition (indicated by the illustratedstate vectors). If a state transition does not complete within athreshold number of cycles of a dedicated clock signal provided to theguardian circuit module, the guardian circuit module asserts a resetsignal, which causes the FSM characterized in the state diagram 400 toexecute operation 404, such that the FSM transitions to state 0, S0.

Referring back to FIG. 3 , as demonstrated in FIGS. 1 and 2 , theguardian circuit module 304 is implemented with a simple design. Infact, as demonstrated in FIG. 4 , the guardian circuit module 304 haslogic that is simpler than the FSM 308. Additionally, as demonstrated inFIG. 4 , assertion of the reset signal, RESET causes the FSM to enter asafe state (state 0, S0, in FIG. 4 ), if FSM 308 does not complete astate transition within a threshold number (e.g., five) of clock cyclesof the guardian clock cycle, GUARD CLK.

FIG. 5 illustrates a flowchart of an example method 500 for monitoringoperations of an external circuit. In some examples, the method 500 isimplemented by a circuit, such as the guardian circuit module 100 ofFIG. 1 , the guardian circuit module 200 of FIG. 2 , and/or the guardiancircuit module 304 of FIG. 3 . In some examples, the external circuit isimplemented with the external circuit 103 of FIG. 1 . In such asituation, the external circuit is implemented as an FSM.

At 505, the circuit monitors the operations of the external circuit. At510, a determination is made by a guard trigger circuit (e.g., the guardtrigger 104 of FIG. 1 ) of the circuit as to whether a transition in theexternal circuit is in progress. In some examples, the determination at510 is made based on a comparison of signals, such as the state signal,STATE, the next state signal, NEXT STATE, the brownout signal, BROWNOUTand/or the guard enable signal, GUARD ENABLE of FIG. 1 . In otherexamples, other combinations and/or subsets of signals are employable toenable the detection at 510. If the determination at 510 is negative(e.g., NO) the method 500 returns to 505). If the determination at 510is positive (e.g., YES), the method 500 proceeds to 515.

At 515, the guard trigger circuit of the circuit asserts a first signal,such as the trigger signal, TRIGGER_N of FIG. 1 . At 517, adetermination is made as to whether the transition is still in progress.If the determination at 517 is negative (e.g., NO), the method 500proceeds to 519. If the determination at 517 is positive (e.g., YES),the method proceeds to 520. At 519, the first signal is de-asserted,indicating that the transition has completed, and the method 500 returnsto 505.

At 520, a determination is made as to whether the first signal has beenasserted for a first predetermined number of clock cycles of a dedicatedclock signal. The determination at 520 is made, for example, by a resetsynchronizer circuit of the circuit, such as the reset synchronizer 108of FIG. 1 . If the determination at 520 is negative (e.g., NO), themethod 500 returns to 517. If the determination at 520 is positive(e.g., YES), the method 500 proceeds to 525.

At 525, the reset synchronizer circuit of the circuit asserts a secondsignal, such as the synchronization signal, SYNC_N of FIG. 1 . At 527,another determination is made as to whether the transition is still inprogress. If the determination at 527 is negative (e.g., NO), the method500 proceeds to 529. If the determination at 527 is positive (e.g.,YES), the method proceeds to 530. At 529, the second signal isde-asserted, indicating that the transition has completed, and themethod 500 returns to 519.

At 530, a determination is made as to whether the second signal has beenasserted for a second predetermined number of clock cycles of thededicated clock signal. The determination at 530 is made, for example,by a timeout circuit of the circuit, such as the timeout circuit 116 ofFIG. 1 . If the determination at 530 is negative (e.g., NO), the method500 returns to 527. If the determination at 530 is positive (e.g., YES),the method 500 proceeds to 535.

At 535, the timeout circuit of the circuit asserts a third signal, suchas the timeout signal, TIMEOUT of FIG. 1 . At 540, yet anotherdetermination is made as to whether the transition is still in progress.The determination at 540 is made, for example, by a reset requestorcircuit of the circuit, such as the reset requestor 112 of FIG. 1comparing the first signal and the third signal. For instance, if thereset requestor determines that the third signal and the first signalare both asserted concurrently, the determination at 540 is positive,and the determination at 540 is negative if either (or both) the thirdsignal or the first signal are de-asserted. If the determination at 540is negative (e.g., NO), the method 500 proceeds to 542. If thedetermination at 540 is positive (e.g., YES), the method 500 proceeds to545. At 542, the third signal is de-asserted and the method 500 returnsto 529. At 545, the reset requestor asserts an output signal, such asthe reset signal, RESET of FIG. 1 . In some examples, the output signalis provided to an input node of a controller that includes the externalcircuit, such as the controller 312 of FIG. 3 .

Modifications are possible in the described examples, and other examplesare possible, within the scope of the claims.

What is claimed is:
 1. A circuit comprising: a guard trigger circuitcomprising: a first input node adapted to be coupled to a first statesignal; a second input node adapted to be coupled to a second statesignal; and an output node adapted to provide a trigger signal, whereinthe guard trigger circuit is adapted to: determine a state transition inprogress based on the first state signal and the second state signal;and provide the trigger signal based on whether the state transition isin progress; a reset synchronizer circuit comprising: an input nodecoupled to the output node of the guard trigger circuit; a clock nodeadapted to be coupled to a clock signal; and an output node adapted toprovide a synchronization signal based on the trigger signal; a timeoutcircuit comprising: an input node coupled to the output node of thereset synchronizer circuit; a clock node adapted to be coupled to theclock signal; and an output node adapted to provide a timeout signalbased on the synchronization signal; and a reset requestor circuitcomprising: a first input node coupled to the output node of the guardtrigger circuit; a second node coupled to the output node of the timeoutcircuit; and an output node adapted to provide a reset signal based onthe trigger signal and the timeout signal.
 2. The circuit of claim 1,wherein the guard trigger circuit comprises a third input node adaptedto be coupled to a brownout signal, and wherein the guard triggercircuit is adapted to provide the trigger signal based on the brownoutsignal.
 3. The circuit of claim 2, wherein the guard trigger circuitcomprises a fourth input node adapted to be coupled to a guard enablesignal, and wherein the guard trigger circuit is adapted to provide thetrigger signal based on the guard enable signal.
 4. The circuit of claim1, wherein the guard trigger circuit comprises a gate that de-asserts astate transition signal if the first state signal and the second statesignal are asserted and asserts the state transition signal if the firststate signal or the second state signal is de-asserted.
 5. The circuitof claim 4, wherein the guard trigger circuit asserts the trigger signalon the output node of the guard trigger circuit in response to the statetransition signal being asserted.
 6. The circuit of claim 5, wherein thereset synchronizer circuit comprises a shift register that asserts thesynchronization signal at the input node of the timeout circuit inresponse to the state transition signal being asserted for apredetermined number of cycles of the clock signal.
 7. The circuit ofclaim 6, wherein the reset synchronizer circuit comprises cascaded Dflip-flops, and the state transition signal is coupled to a preset inputof the cascaded D flip-flops.
 8. The circuit of claim 6, wherein thepredetermined number of cycles of the clock signal is a firstpredetermined number of cycles of the clock signal, and the timeoutcircuit comprises a shift register that asserts a timeout signal at thesecond input node of the reset requestor circuit in response to thesynchronization signal being asserted for a second predetermined numberof cycles of the clock signal.
 9. The circuit of claim 8, wherein thetimeout circuit comprises cascaded D flip-flops, and the synchronizationsignal is coupled to a clear input of the cascaded D flip-flops of thetimeout circuit.
 10. The circuit of claim 8, wherein the reset requestorcircuit asserts the reset signal on the output node of the resetrequestor circuit in response to detecting the trigger signal coupled tothe first input node of the reset requestor circuit being asserted andthe timeout signal coupled to the second input node of the resetrequestor circuit being asserted concurrently.
 11. The circuit of claim10, wherein the reset signal is adapted to be coupled to an input of acontroller.
 12. The circuit of claim 11, wherein the clock signal isindependent of a system clock signal employed to control operations of afinite state machine and the clock signal has a counter interval that islonger than a counter interval of the system clock signal.
 13. A methodcomprising: asserting a trigger signal in response to detecting that atransition in a circuit is in progress; asserting a synchronizationsignal in response to detecting the trigger signal being asserted for afirst number of cycles in a clock signal; asserting a timeout signal inresponse to detecting the synchronization signal being asserted for asecond number of cycles in the clock signal; and asserting a resetsignal in response to detecting the timeout signal and the triggersignal being asserted concurrently.
 14. The method of claim 13, whereinthe circuit comprises a finite state machine (FSM), the method furthercomprising inhibiting assertion of the trigger signal in response todetecting a brownout signal indicating that the FSM is being reset to aninitial state.
 15. The method of claim 14, the method further comprisesreceiving a first state signal characterizing a state of the FSM and asecond state signal characterizing a next state of the FSM, wherein adifference in the first state signal and the second state signal isindicative of the transition in the FSM is in progress.
 16. The methodof claim 15, further comprising de-asserting the trigger signal inresponse to determining that the first state signal and the second statesignal indicate that a state transition of the FSM is not in progress orthe state transition has been completed.
 17. The method of claim 14,wherein the clock signal has a counter interval that is longer than asystem clock signal employed for controlling operations of the FSM. 18.The method of claim 13, wherein the first number of clock cycles of theclock signal is different than the second number of clock cycles of theclock signal.
 19. The method of claim 13, further comprising generatingthe clock signal.
 20. The method of claim 13, wherein the reset signalis coupled to an input node of a controller that comprises the circuit,the method further comprising resetting the circuit in response to thereset signal being asserted.